Republican Debate Spotlight Intensifies as Iowa Caucuses Near

Thursday night’s Republican presidential debate in North Charleston, S.C., will be the sixth of the primary campaign, but only the next to last before the Iowa caucuses on Feb. 1. And as Newt Gingrich can attest, face-offs held in the final weeks and days before the nominating contests tend to matter more. And the attacks tend to get rougher.

Here are some likely clashes to watch for in the main debate, to be shown on Fox Business Network at 9 p.m. Eastern time.

Trump vs. Cruz

Donald J. Trump, through a series of news media appearances, has slowly but ingeniously generated and encouraged questions about Senator Ted Cruz’s eligibility to be president of the United States.

Mr. Cruz, the Texas senator, was born in Canada to a Cuban father but to an American mother, which legal experts say clears any constitutional hurdle. But a number of new polls indicate that the attacks might be taking a toll on Mr. Cruz, who narrowly leads Mr. Trump in Iowa.

But while Mr. Trump spent four minutes on the subject at an Iowa rally last week, he has shied away from delivering some of his more biting campaign-trail attack lines when face to face with his rivals in previous debates. And he told CNN that he had no plans to bring up whether Mr. Cruz is a “natural-born citizen” on Thursday night.

Donald Trump during a campaign rally in Pensacola, Fla., on Wednesday.CreditMichael Snyder/Associated Press

It remains to be seen whether Mr. Cruz will forcefully assail Mr. Trump’s command of foreign affairs, his “New York values” or anything else about him in a nationally televised debate.

But Mr. Trump may not need to mention Canada at all. The New York Times’s report that Mr. Cruz, who spent years railing against bank bailouts, received up to $1 million in undisclosed loans from Goldman Sachs and Citibank for his first Senate campaign, could be a brutal weapon for any rival to use against him.

Cruz and Christie vs. Rubio

Will Gov. Chris Christie of New Jersey and Mr. Cruz team up against Senator Marco Rubio of Florida? And will Mr. Rubio be ready for them?

For weeks, Mr. Christie seemed to be having a resurgence in New Hampshire. Then an outside group supporting Mr. Rubio aimed a blistering attack ad at him there. Mr. Christie responded viscerally, saying that Mr. Rubio was too weak and ineffective to take on Hillary Clinton and suggesting that she would “cut his heart out” in a general election debate.

Mr. Rubio also strayed into a minefield with Mr. Cruz after trying to deflect criticism of his own record on immigration by suggesting that the two shared the same positions on it. Mr. Cruz has retaliated by pointing repeatedly to Mr. Rubio’s work with Senator Chuck Schumer of New York, who is loathed by conservatives, on a failed effort to overhaul the nation’s immigration laws, which conservatives loathed even more.

In both instances, Mr. Rubio appeared to wander into a fight that became more intense than he was ready for. And his allies have generally been better at complaining about the attacks against him than at answering them effectively.

Mr. Rubio has had weeks to prepare for the attacks he can expect onstage Thursday, however. How well he counterpunches will be closely watched.

Bush vs. Trump, or Someone Else?

In recent weeks, former Gov. Jeb Bush of Florida has emerged as Mr. Trump’s chief critic, portraying himself as the only contender gutsy enough to stand up to him.

But Mr. Trump is not Mr. Bush’s most immediate concern. The “super PAC” supporting Mr. Bush, Right to Rise USA, has been firing away with direct mail pieces at Mr. Christie in New Hampshire. And Mr. Bush has raised questions about Mr. Christie’s record as the steward of New Jersey’s finances.

Like Mr. Christie, Mr. Rubio and Gov. John R. Kasich of Ohio, Mr. Bush is vying for support from right-of-center voters who are turned off by Mr. Trump and Mr. Cruz. He badly needs a breakthrough debate moment.

Should he successfully go after someone who poses a direct threat to his strategy, he could steal attention heading into the final stretch before Iowa and New Hampshire.

Trump vs. Himself

In the five previous debates, Mr. Trump has been flanked by a wide array of rivals on either side. At times, he seemed content to disappear while others onstage addressed the intricacies of policy — the sort of matters that Mr. Trump has sometimes appeared to bluff his way through.

But in South Carolina, just seven candidates will participate in the main event. That means more microphone time for Mr. Trump, but less opportunity to hide.

And the moderators, from Fox Business, were heavily focused on policy in the last debate they oversaw, in Wisconsin in November. Their questions then were dense with details and did not often lend themselves to glib answers.